The main improvement that Servo's architecture brings over things like Oilpan (and, it sounds like, some other browser engines too) is that our GC is single-threaded, despite the browser itself being highly multithreaded, and it's precise instead of conservative. (Well, to be more exact, it will be precise when we upgrade our copy of SpiderMonkey. Also note that Oilpan is only mostly conservative, in that it can do precise collections if no C++ is on the stack.)
Single-threaded GC is nice because it doesn't have to stop any other threads in order to run; it also tends to reduce the size of the heap that must be scanned. For example, while script is collecting garbage, the layout and painting threads can run unimpeded. Of course, using a single-threaded GC in a multithreaded app means you have to be extremely careful in order to avoid the GC freeing things you don't want to be freed, and that is why it is seldom done. Our trick is to use the Rust type system to enforce that this is done correctly at compile time, which is something that is not possible in C++.
